I personally don't think your runes setup is worth it. Getting mana/mana regen in-game is fairly cheap. As a caster I believe you need HP quints..I'm not sure why you have 3 of your rune types devoted to mana. I just don't think it's worth it.

And nashor's tooth? No idea why you recommend that. There is absolutely no reason why nashor's would be better on a caster than frozen heart if you are discussing CD reduct. Also your deathfire and your nashor's would not stack, making the passive's redundant.

thanks for youre reply, youre right about the tanks - hp just flies of m. I tested all this in game except for the huge cooldown. I just found out that 40% is the maximum CD reduction you can get, and I waisted my entire night from 3 am to 6 am writing this...

i like it for the most part, but a malz should never be running with a mejai's. unless you are up against complete noobs and they fear you, its a complete waste. maintaining those stacks is difficult because of malz's lack of escape ability. flash can only get you so far. IMO ghost > flash in this situation. on top of that argument null zone does a percent of the enemy champs max hp as dmg. as long as you have spell pen tanks explode when you stun them on your zone :p without the abundance of AP. you shouldnt need more than 300 end game to still nuke stuff.

I chose Nashor for the extra AP that it gives. Frozen heart gives a usefull mana addition but in my opinion wether its 42% with no armor or 62% with armor will not help if he gets attacked in team fights. Soul shroud is a nice idea for the hp, but i believe it only gives youre teammates cooldown reduction - is this correct? I hope this explains, thanks for reading my build man!

EDIT: I've gotten Frozen Heart as 6th item for its mana

I don't see a use for the Nashor's Tooth. I feel if you wanted that extra cooldown you'd be better off getting a different item like a Frozen Heart or a Soul Shroud.